If you just want to switch over to a second filter, there are a couple
of different valve combinations that will do the job. However, if you
want to replace a filter element while the engine is running, you have
to make sure that air cannot be pulled into the engine fuel line via
the open filter. It's also good to have a system that is simple to
understand if someone who is new to the boat has to operate the valves.
On my boat I use a tee at each end of the filter bank, with on-off ball
valves at the inlet and outlet of each filter. Pretty obvious setup for
a stranger to figure out, and you can completely isolate whatever
section of the system you are working on.
You also have to figure out a way to re-prime the new filter with fuel.
On the bigger Racors with the T-handle you can pour fuel in the top
from a camping-type fuel container. Some of the smaller filters come
with a priming pump.
Two filters is a great idea, and saved me in a tight spot one night.
Well worth the several hundred in filters and plumbing!
Cheers
John